1. Field of the Invention
The invention relates generally to the field of illuminators for small displays. More particularly, the invention relates to compact imaging systems using folded optical paths to illuminate a small reflective display.
2. Description of the Related Art
Liquid Crystal on Silicon (LCoS) micro-displays such as the CMD8X6D and CMD8X6P available from Zight Corporation of Boulder Colo. provide great advantages for compact near-eye applications. LCoS micro-displays produce a high resolution image by changing the polarization state of incident light. In the dark state, a pixel reflects light with substantially no change in polarization. In the bright state, the pixel rotates the polarization state of reflected incident light to the corresponding orthogonal state. By illuminating the display with polarized light and then filtering out nearly all reflected light of that polarization, the display image can be viewed by the human eye. Other miniature displays use either polarization effects or reflectivity changes to produce an image.
Typically, the display is illuminated with pulsed red, green, and blue light while the display is synchronized to the pulsed light source to reflect the appropriate color component of the image. The rapidly alternating red, green, and blue images are blended in human perception to form the full-color image of the display. However, the display can also be illuminated with monochromatic light for data or targeting displays. Such displays are used, for example in helmet, windshield, and visor projection systems as well as in small portable headsets and handsets for private display viewing and for virtual reality systems.
A typical illumination and eyepiece system using pulsed LEDs to illuminate the display and a polarizing beam splitter to conduct the reflected bright light to a viewer is shown, for example, in U.S. Pat. No. 6,038,005 to Handschy et al, FIG. 18A. In that patent, the light from the pulsed LEDs is diffused, then collimated by a Fresnel lens and directed to a polarizing beam splitter cube. The cube reflects polarized light from the LEDs to the micro-display. The polarized light is reflected from the micro-display back toward the beam splitter cube. If the polarization state of the light has been rotated then it will pass through the beam splitter cube to an eyepiece that images the reflected light for the viewer. If the light is reflected from the micro-display without a change in polarization, then it will be reflected by the beam splitter cube away from the viewer and back toward the LED source.
A more compact frontlight suitable for use with reflective displays such as an LCoS micro-display is shown in U.S. Pat. No. 6,005,720 to Watters et al. This design has a cemented prism block with an internal polarizing beam splitter 26, 14, 30. A Fresnel element 22, used as a collimator and a pre-polarizer 24, filters out all but the S-polarized light. However, this prism design is very bulky and results in a thicker, deeper, and heavier overall form factor. In addition additional optical elements 62 are required to give a wider field of view for the user. Secondly, the design in the above-mentioned patent provides a telecentric pupil. A significantly non-telecentric design, for both the frontlight and the eyepiece can significantly improve compactness.
A method and apparatus are described that provide an enhanced illuminator for a micro-display. In one embodiment, the invention is an illuminator for a reflective display panel having a light source and a prism. The prism has a first substantially planar face proximate to the light source and to the display panel, so that light propagates from the light source into the prism through the first face and is redirected to the display panel through the first face.
A second face of the prism opposite the light source so that light reflected from the display panel impinges on the second face to propagate to imaging optics. The illuminator may also include an auxiliary prism optically coupled to the second face of the first prism to correct for astigmatism of the reflected light from the display panel and a beam splitter between the first prism and the auxiliary prism, wherein light from the light source is reflected by the beam splitter to the imaging optics.
In another embodiment, the invention is an illuminator for a reflective display panel having a light source, a converging optical element to receive light from the light source and a prism. The prism has a first face to receive light from the converging optical element, the prism receiving light from the light source and redirecting it as diverging light to the display panel, and a second face opposite the display panel for receiving light reflected from the display panel and propagating the reflected light to imaging optics.
Other features of the present invention will be apparent from the accompanying drawings and from the detailed description that follows.